Genetics Notes
Genetics – the study of heredity
Heredity – passing of traits from parent of offspring
Trait – any characteristic that can be passed from parent to offspring
*Summarize the outcomes of Gregor Mendel’s experimental procedures*
Gregor Mendel was an Austrian monk who lived in the 1800s and worked with pea plants.
Gregor Mendel is considered the father of genetics.
Mendel’s Experiment:
1. Mendel started with pure plants.
Pure plants will produce only one characteristic for a trait.
Mendel produced pure plants by repeated self-pollination of plants.
Self-pollination is the fertilization of an egg by pollen from the same plant.
Cross-pollination is the fertilization of an egg by pollen from another plant.
- Mendel cross-pollinated pure plants with two different characteristics for a trait.
Cross-pollination involves the transfer of pollen from one plant to another to fertilize the egg. He called these two pure plants the P1 Generation (parental generation).
Example: Mendel cross-pollinated a pure tall plant with a pure short plant.
- The offspring of the P1 Generation he called the F1 Generation (first filial
generation) (Filial means having the relationship of children to parents). He then
self-pollinated one the F1 Generation plants.
- The offspring of the F1 Generation he called the F2 Generation (second filial
generation).
Mendel’s Results:
1. For all the traits only 1 characteristic would appear in the F1 generation and always
the same characteristic. Example: Pure Tall X Pure Short = all the offspring tall
- Self-pollination of the F1 Generation to produce the F2 Generation always resulted
in the missing characteristic reappearing in the F2 Generation in a predictable
ratio. The ratio was always 3:1 with one being the missing characteristic.
Example: 3 offspring tall and 1 short
Mendel’s Conclusions:
- Hereditary traits are passed from parents to offspring during reproduction
- For each trait there are two genes. One from each parent.
Alleles are different forms of the same gene. Example: tall and short
Heterozygous – organisms that have 2 different alleles for a trait
Homozygous – organisms that have 2 of the same alleles for a trait
- Principle of Dominance – when the two genes for a trait are different one of the
genes will hide the other. The gene that hides another gene is called the dominant
gene. The hidden gene is called the recessive gene.
- Law of Segregation – the two genes for the trait are separated
This occurs during anaphase I of meiosis. Mendel realized this 25 years before meiosis was discovered.
- Law of Independent Assortment – genes are separated and distributed
independently to gametes
Exception: Linkage – genes located close together on the same chromosome are
inherited together (Locus is the term for gene position on the chromosome.)
Reasons for Mendel’s Success
- Mendel did thousands of crosses
- The genes Mendel studied were on different chromosomes so linkage did not interfere
- The genes were dominant and recessive.
- The traits were controlled by single genes.
*Differentiate among the laws and principles of inheritance*
1. Principle of Dominance – in the two different forms or alleles of a gene one may hide the other gene. The hidden gene is called the recessive gene and the dominant gene is the one that hides the recessive gene.
2. Principle of Segregation – during meiosis the two genes for a trait are separated. This occurs during Anaphase I. One of these genes came from the mother and one of these genes came from the father.
3. Law of Independent Assortment – the genes for different traits are inherited independently of one another except for those genes that are linked on the same chromosome
*Use the laws of probability and Punnett squares to predict genotypic and phenotypic ratios*
Probability – the likelihood that an event will happen
Probability = the number of times a particular event occurs
the number of possible events
Rule of Independent Events – previous events do not affect the probability of the later
occurrences of an event
Example: after 10 heads in a row the 11th toss still has
a 50-50 chance of being heads
Product Rule – the probability of independent events happening together equals the
product of the probabilities of the events occurring separately
Punnett Square – chart used to determine the probabilities of the genotypes and
phenotypes of the offspring produced by various parents
Genotype – the actual genes present in an organism
Phenotype – the appearance of the organism; the outward expression of the genotype
Homozygous – the two genes for a trait are the same
Heterozygous – the two genes for a trait are different
Rules for Punnett Squares
- The first letter of the dominant trait is used to represent the trait
- A capital letter is used to represent the dominant gene
- A lower case letter is used to represent the recessive gene
- Capital letters are written first
- Like letters are written together
Monohybrid Crosses – involve only one trait
- Homozygous Dominant X Homozygous Recessive Cross
A homozygous tall pea plant is cross-pollinated by a homozygous short pea plant.
Tall is dominant and short is recessive
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
- Heterozygous X Heterozygous Cross
A heterozygous tall pea plant is self-pollinated by a heterozygous tall.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
- Homozygous dominant X Heterozygous
A round seed shape pea plant is cross-pollinated by a heterozygous round seed shape pea plant.
Wrinkled is the other seed shape.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
- Homozygous recessive X Heterozygous
A terminal flower position pea plant is cross-pollinated by a heterozygous axial flower position pea plant.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
- Homozygous dominant X Homozygous dominant
A homozygous yellow seed color pea plant is self-pollinated by a homozygous yellow seed color pea plant.
Yellow is the dominant seed color in pea plants
Green is the recessive seed color in pea plants.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
- Homozygous recessive X Homozygous recessive
A homozygous white seed coat color pea plant is self-pollinated by a homozygous white seed coat color pea plant.
Gray is the dominant seed coat color.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
*Examine different modes of inheritance*
1. Sex-linked traits – Genes carried on the X chromosome but not the Y
Comparing an X chromosome to a Y chromosome the Y is missing
a leg. The sex-linked trait genes are found on this missing leg. In
humans males are XY and females are XX, so mothers pass on sex-
linked traits to their sons.
Color blindness and hemophilia are sex-linked traits in humans.
Both color blindness and hemophilia are recessive traits.
Sample Punnett square:
A woman who carries the gene for color blindness but has normal vision marries a man with normal vision.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
2. Codominance – the two genes for a trait are not dominant or recessive. They are both
expressed at the same time.
Human blood type is an example codominance.
Type A blood is codominant to Type B blood
Type O blood is recessive to both Type A blood and Type B blood
Type A blood genotypes: IAIA Homozygous Type A
IAi Heterozygous Type A
Type B blood genotypes: IBIB Homozygous Type B
IBi Heterozygous Type B
Type O blood genotype: ii Homozygous Type O
Sample Punnett square:
A heterozygous Type A woman marries a heterozygous Type B man.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
3. Incomplete dominance – The two genes for a trait are not dominant or recessive but
the two traits blend.
Flower color in four o’clocks and snap dragons is an
example of incomplete dominance
Sample Punnett square:
A red snap dragon is cross-pollinated by a white snap dragon.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
Sample incomplete dominance problem:
A pink snap dragon is self-pollinated.
Male genotype: Offspring Offspring
Female genotype: genotypes phenotypes
- Multiple alleles – having more than two alleles for a trait.
Human blood type is an example of multiple alleles.
There are three alleles, a gene for A type blood, a gene for B
type blood, and a gene for O type blood.
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